The use of ophthalmic lenses for the correction of ametropia is well known. For example, multifocal lenses, such as progressive addition lenses ("PAL's"), are used for the treatment of presbyopia. The progressive surface of a PAL provides far, intermediate, and near vision in a gradual, continuous progression of vertically increasing dioptric power from far to near focus, or top to bottom of the lens.
PAL's are appealing to the wearer because PAL's are free of the visible ledges between the zones of differing dioptric power that are found in other multifocal lenses, such as bifocals and trifocals. However, an inherent disadvantage in PAL's is unwanted astigmatism, or astigmatism introduced or caused by one or more of the lens' surfaces. In hard design PAL's, the unwanted astigmatism borders the lens channel and near vision zone. In soft design PAL's, the unwanted astigmatism extends into the distance vision zone. Generally, in both designs the unwanted lens astigmatism at or near its approximate center reaches a maximum that corresponds approximately to the near vision dioptric add power of the lens.
Many PAL designs are known that attempt to reduce unwanted astigmatism with varying success. One such design is disclosed in U.S. Pat. No. 5,726,734 and uses a composite design that is computed by combining the sag values of a hard and a soft PAL design. The design disclosed in this patent is such that the maximum, localized unwanted astigmatism for the composite design is the sum of the contributions of the hard and soft designs areas of maximum, localized unwanted astigmatism. Due to this, the reduction in the maximum, localized unwanted astigmatism that may be realized by this design is limited. Therefore, a need exists for a design that permits even greater reductions of maximum, localized unwanted astigmatism than in prior art designs.